System for conducting heat from an electrode rotating in a vacuum

ABSTRACT

Heat is conducted from an electrode moving in an evacuated enclosure and subject to heating by an electron beam by interposing a film of liquid metal between stationary and moving parts of the thermal path between the moving electrode and a stationary metal surface and cooling the stationary metal surface.

United States Patent Houston 1451 Sept. 26, 1972 [54] SYSTEM FORCONDUCTING HEAT FROM AN ELECTRODE ROTATING IN A VACUUM [72] lnventor:John M. Houston, Schenectady,

Assignee: General Electric Company Filed: June 28, 1971 Appl. No.:157,483

313/330 Int. Cl ..H0l' i 35/10 Field of Search ..313/60, 330, 11, 46

[56] References Cited UNITED STATES PATENTS 3,546,511 12/1970 Shimula..3l3/60 Primary Examiner-Roy Lake Assistant Examiner-Darwin R.Hostetter Attorney-John F. Ahem et a1.

57 ABSTRACT Heat is conducted from an electrode moving in an evacuatedenclosure and subject to heating by an electron beam by interposing afilm of liquid metal between stationary and moving parts of the thermalpath between the moving electrode and a stationary metal surface andcooling the stationary metal surface.

13 Claims, 5 Drawing Figures PATENTEDszrzs r912 SHEET 2 0F 5 INVENTORJOHN M. HOUSTON BY 4! 0. 2W2, A WM HIS ATTORNEY PATENTEU I972 3.694.685

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HIS ATTORNEY PATENTED 8P26 I972 SHEET 5 0F 5 INVENTOR JOH M. HOUSTON BYAM/Z] HIS ATTORNEY SYSTEM FOR CONDUCTING HEAT FROM AN ELECTRODE ROTATINGIN A VACUUM My invention relates to electron discharge devices, and inparticular, to such devices which employ a rotating electrode which issubjected to a high intensity beam so that the electrode is heated tohigh temperatures.

Rapidly rotating electrodes are used in a variety of high vacuumdevices, one principal user being the rotating anode X-ray tube which isthe dominant device in current installations of medical X-ray equipment.Such rotating electrodes are subjected to high intensity electron beamswhich cause the electrode to heat rapidly. Conventionally, the rotatinganodes are cooled by radiation, a process which can transfer only asmall amount of heat unless the electrode is permitted to reach veryhigh temperatures during its operation. Thus, in an X-ray tube thesteady-state power dissipation through transfer of heat by radiation fora rotating anode is a mere 500 watts even when the anode is operating atbright red heat. In contrast, peak dissipation, or energy of theelectron beam, is in excess of 100,000 watts. Even though the period ofelectron bombardment of the electrode for generation of X-rays isextremely short, the problem of overheating the anode limits the averagepermissible X-ray intensity. In addition to the fact that the anode isheated to excessive temperatures, the bearings in the structuresupporting the anode are subjected to overheating because they are inthe thermal path through which heat is conducted from the anode to theexterior of the tube.

The primary object of my invention is to provide new .and improvedapparatus and methods for cooling electrodes rotating in a vacuum.

Another object of my invention is to provide new and improved means forcooling rotating electrodes which permits increasing the average poweroutput of the device employing such electrodes.

Still another object of my invention is to provide methods and apparatusfor cooling the rotating anode of an X-ray tube which also reduces thevibrations of the anode.

In its broadest aspect, my invention consists in conducting heat from amoving electrode in an evacuated enclosure, which electrode is subjectedto heating by an electron beam, by interposing a film of liquid metalbetween stationary and moving parts of a thermal path between the movingelectrode and a stationary metal surface which can be cooled byconventional cooling means external to the device employing the movingelectrode. An annular gap in the supporting structure is filled with alow vapor pressure liquid metal which conducts heat from the movingelectrode to a hollow stationary metal cylinder, the stationary cylinderbeing cooled by conventional means such as a circulating fluid. Inaddition to conducting heat from the electrode, the liquid metal alsodamps vibrations normally present in rotating anode X-ray tubes.

These and other important objects and advantages of the invention willbecome apparent as the invention is understood from the followingdescription which, taken in connection with the accompanying drawings,discloses preferred embodiments of the invention. In the drawings,

FIG. 1 is a vertical view, partly in section, of a rotating anode typeof X-ray tube embodying the invention;

FIG. 2 is an enlarged view of a portion of the rotating anode of FIG. 1which illustrates details of the cooling system for rotating electrodes;

FIG. 3 illustrates another form of a rotating anode for an X-ray tubeembodying my invention;

FIG. 4 illustrates another bearing structure for a rotating anode for anX-ray tube embodying my invention; and

FIG. 5 illustrates still another form of a rotating anode for and X-raytube embodying my invention.

The rotating electrode structure shown in FIGS. 1 and 2 comprises ananode 1 mounted at one end of armature 2 comprising an outer electricalrotor portion 3 and a hollow central shaft 4 supported by bearings 5.The electrode structure is contained within an evacuated region formedby a glass envelope 6. Also contained within envelope 6 is a cathode 7.A field coil 8 surrounds the portion of envelope 6 which contains rotor3 and supplies the electrical field for rotating the armature structure.A hollow metal cylinder 9 is positioned between rotor 3 and cylinder 4and provides a stationary support for bearings 5. The lower portion ofcylinder 9 is sealed to a reentrant portion 10 of the glass envelope.Along the axis of concentric members 3, 4 and 9, there is located afixed, hollow metal cylinder 11 having an enlarged portion 12 at itsupper end. Cylinder 11 is supported by cylinder 9, having its lower endsealed to a depending portion 13 of cylinder 9 by a weld 14. A metalsleeve 15 is slipped within the central bore of shaft 4 and has itsbottom end welded to cylinder 4 at point 16. Sleeve 15 carries aplurality of traps 17 on its inner surface.

In accordance with my invention, the annular gap between the enlargedportion 12 of cylinder 11 and the inner surface of hollow shaft 4 isfilled with a low vapor pressure liquid metal 18 illustrated in FIG. 2.This liquid metal is initially placed in either uppermost trap 17 or theregion 20 between the upper end of portion 12 and a transverse portion21 of shaft 4. A duct 22 in enlarged portion 12 facilitates evacuationof region 20 during initial processing of the X-ray tube. Fixed metalcylinder 11 and enlarged upper portion 12 which are rigidly attached tothe bearing structure of the tube are cooled from sources external tothe X-ray tube. As shown in FIG. 2, one such method of cooling tube 11comprises a conduit 23 centrally located within cylinder 11 and throughwhich a cooling fluid 24, such as oil or water, is circulated. The outersurface of enlarged portion 12 and the inner surface of the upper end ofshaft 4 are closely spaced so that the annular gap 25 between thesesurfaces has a width of a few thousandths of an inch, i.e., in the rangeof 0.0005 to 0.030 inch.

The liquid metal 18 contained within gap 25 may be any suitable alloy oflow vapor pressure metals and preferably is liquid at the ambienttemperature of the apparatus. One suitable metal possessing a low vaporpressure and which is liquid at room temperature comprises an alloy ofgallium, indium and tin. While a metal having a melting point somewhatabove room temperature could be used, preheating of the anode and thecoolant to above the melting point of the liquid metal would be requiredbefore the device may be used. Hence, it is desirable that the metal beliquid at room temperature. Additionally, the coolant metal employedshould have a low vapor pressure at the temperature at which the X-raytube is baked-out during its evacuation to prevent distilling of themetal around the inside of the X-ray tube. During such evacuation, thetube is typically heated to a temperature of the order of 450 C.Consequently, it is desirable that the metal employed have a low vaporpressure at this temperature. Additionally, the metal employed shouldhave a low viscosity so that the viscous drag on the rotating electrodedoes not cause an appreciable increase in the power required to producerotation.

In operating the X-ray tube of FIGS. 1 and 2, when anode 1 begins torotate, centrifugal force causes the liquid metal 18 contained in trap17 to migrate rapidly into gap 25. Since the outer surface, rather thanthe inner surface, is the rotating surface, centrifugal forces on theliquid metal tend to drive the liquid metal from trap 17 into annulargap 25. During rotation the surface of the liquid metal assumes a shapesimilar to that shown in region 20 of FIG. 2. When rotation of the anodeceases, while capillary attraction will tend to maintain the liquidmetal in annular gap 25, any of the metal which does not remain withinthe gap is caught by one of the traps 17 During operation of the X-raytube when anode 1 is heated to a high temperature, athermally-conductive path for cooling the anode is provided by the shortthermal path from anode l to the upper portion of hollow cylinder orshaft 4 which supports the anode and through the layer of liquid metalin annular gap 25 to enlarged portion 12 of central cylinder 11. Heattransferred from the anode through this short, direct, thermalconductive path is carried away by circulating fluid 24. The film ofliquid metal in gap 25 is thus a direct thermally-conductive pathbetween rotating shaft 4 and stationary cylinder ll.'The presence ofthis direct thermal conductive path in proximity to the heated anodecauses rapid transfer of heat from the anode to the external coolingmeans and prevents or largely reduces transfer of heat to the rotor 3and bearings 5. The arrowsl9 indicate the direction of the heat flowfrom shaft 4 through the liquid metal in gap 25 to enlarged portion 12.

When gallium is one constituent of the liquid metal used for cooling therotating electrode, certain care should be observed in choosing metalsincontact with the liquid metal because gallium tends to alloy readilywith many other metals. The metals rhenium, tungsten, molybdenum,tantalum, niobium, zirconium, and stainless steel, however, are amongthose which do not alloy readily with gallium up to temperatures of theorder of 400 C. a

In the embodiment of my invention shown in FIG. 3,

7 central shaft member 30 with its enlarged upper portion 31, inaddition to functioning to conduct heat from anode 1, also supports twobearings 32, which in turn support hollow shaft 33, anode l and rotor 34being mounted on shaft 33. In this structure, a film of liquid metal ismaintained in an annular gap 35 between member 31 and the inner surfaceof the upper portion does not react with the liquid metal. Where thatliquid metal comprises an alloy of gallium, indium, and tin, the ballbearings may be formed of either steel or aluminum oxide. In thisconstruction, trap 46 located below gap 45 may initially be filled withthe liquid metal which, upon operation of the tube, rises into gap 45.While most of the liquid metal will remain in the annular space 45during inoperative periods of the apparatus, trap 46 receives any whichdrops out of this gap so that it may be returned upon subsequentoperation of the tube.

The structures of FIGS. 3 and 4 may be cooled by circulation of acooling liquid, as is shown in FIG. 2, or alternatively may be cooled bya conventional low temperature heat pipe in which hollow cylinders 30and 43 are filled with a fluid, such as water, mercury, cesium, orrubidium which conducts heat to the exterior surroundings. The use of aheat pipe in this respect is advantageous in that it avoids requirementsof pumps and ancillary equipment for circulating fluids. Additionally,the conduct of heat from the anode l is temperaturedependent. Thus, forexample, if a cesium or rubidium heat pipe is employed, the heat pipeconducts heat strongly only at a temperature above about 350 C. Byconducting heat strongly only at temperatures above this point, thetungsten anode 1 tends to remain at a temperature above 350 C and thusavoids passing through its ductile, brittle temperature during periodsof hollow shaft 33. Trap 36 is located on the inner sur I face of shaft33 and positioned below gap 35.

In the modification of my invention shown in FIG. 4, ball bearings 41,42 positioned between a stationary central cylinder 43 and a rotatingcylinder 44, upon which is mounted rotor member 47, are exposed to theof intermittent use. Such use of a heat pipe, therefore, helps toprevent local thermal stressing and cracking of the tungsten surface ofthe anode.

FIG. 5 illustrates an X-ray tube anode employing my invention in whichthe anode structure is of the type disclosed in US. Pat. applicationSer. No. 602,999 Harold F. Webster, filed Aug. 3, 1970 and assigned tothe assignee of this invention. In this structure, the anode 51comprises a hollow metal member comprising spaced walls 52, 53 and aconnecting peripheral target wall 54. Walls 52, 53 are formed of arefractory metal such as tantalum or niobium, while target wall 54 isformed of a good X-ray target material such as tantalum, tungsten,rhenium, or alloys of these metals. lncluded within the hollow regiondefined by walls 52, 53, 54 is a liquid metal 55 which may consist ofsodium, lithium, cesium, or potassium which is in contact with wall 54.This liquid metal evaporates to provide rapid cooling of the anode andafter condensing is returned to the hot-spots of the anode bycentrifugal force. A metal mesh 56 is attached to the inner surface ofthe hollow member to retain the liquid metal adjacent the target surfacewhile the tube is cooling below the melting point of the liquid metal.Anode 51 is mounted on hollow cylinder 59 which surrounds shaft 58, ballbearings 60 being provided for rotation of shaft 58 about cylinder 59.Enlarged portion 61 on hollow cylinder 59 is closely spaced with respectto the inner surface of shaft 58 adjacent anode 51 to provide an annulargap 62 which is filled with a low vapor pressure liquid metal. Trap 63functions to receive any liquid In the initial operation of my inventionin the various embodiments illustrated, after a short period of rotationof the shaft bearing the rotating electrode the liquid metal rises intothe gap between the electrode supporting shaft and a stationary part ofthe structure. Thereafter, except for extremely long periods of nonuse,a film of liquid metal usually remains in that gap. During operation ofthe device, as the rotating anode heats, heat is transferred through theshort conductive path from the electrode through the liquid metal in theannular gap between that shaft and a stationary member. As pointed outpreviously, preferably a low pressure liquid metal is employed so thatthe electrode may be portion of a device operating in a high vacuum. Theheat transferred or conducted through the thin layer of liquid metal tothe non-rotating portion of the device can then be transferred to theexterior of the device either by convection currents, a heat pipe, orforced cooling means. When my invention is embodied in an X-ray tubeemploying a rotating anode, for example, the rotating anode may beoperated at average power levels of the order of several times greaterthan those currently employed without overheating the anode surface.

While I have shown and described several embodiments of my invention, itwill be apparent to those skilled in the art that many changes andmodifications may be made without departing from my invention in itsbroader aspects and I, therefore, intend the appended claims to coverall such changes and modifications as fall within the true spirit andscope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. The combination in an evacuated electric discharge device comprisinga rotatable electrode structure subject to heating, a supportingstructure for said electrode structure, means for cooling saidsupporting structure, and means for conducting heat from said electrodestructure to said supporting structure comprising a film of liquid metalinterposed between said electrode structure and said supportingstructure.

2. The combination of claim 1 in which the liquid metal is an alloyhaving a low vapor pressure at the normal operating temperature of thedevice.

3. The combination of claim 2 in which the liquid metal is an alloywhich is liquid at ambient temperature.

4. The combination of claim 2 in which the liquid metal has a lowviscosity.

5. The combination of claim 2 in which said liquid metal is an alloy ofgallium, indium and tin.

6. The combination of claim 2 in which said device is an X-ray tube,said electrode structure comprises an anode and a supporting shaft, andsaid supporting structure contains a passageway for receiving a coolingfluid from a source external to said device.

7. The combination of claim 6 in which a bearing is interposed betweensaid rotating electrode structure and said supporting structure and saidliquid metal comprises a material which does not alloy readily with thematerials of said bearing and said rotating structure.

8. The combination of claim 7 in which the liquid metal comprises analloy of gallium, indium and tin,

said be irings are ball bearings and are formed of materia from thegroup consisting of steel and aluminum oxide.

9. The combination of claim 6 in which said supporting structureincludes a centrally positioned hollow cylinder and means for providinga cooling fluid to the interior of said cylinder.

10. The combination of claim 6 in which said supporting structureincludes a centrally positioned hollow cylinder and heat pipe means areincluded in said hollow cylinder.

11. The combination of claim 6 in which said sup-,

porting structure has a closed end which is opposed to said anode toform a region therebetween for receiving liquid metal, said supportingshaft has a liquid metal trap on the inner surface thereof spaced fromsaid supporting structure and said supporting structure includes apassageway for returning liquid metal from said closed region to saidtrap.

12. The method of conducting heat from a moving electrode subject toheating by an electron beam in an evacuated enclosure which comprisesinterposing a film of liquid metal between stationary and moving partsof a thermal path between the moving electrode and a stationary metalsurface and cooling the stationary metal surface.

13. The method of claim 12 in which the moving electrode is the anode ofan X-ray tube, the moving part is a shaft supporting the anode, thestationary part is a bearing structure for the shaft, and which includesthe steps of interposing a film of liquid metal between the shaft andthe bearing structure, trapping any liquid which escapes from the regionbetween the shaft and bearing structure during non-use periods of thetube and returning it to the region during periods of use.

1. The combination in an evacuated electric discharge device comprisinga rotatable electrode structure subject to heating, a supportingstructure for said electrode structure, means for cooling saidsupporting structure, and means for conducting heat from said electrodestructure to said supporting structure comprising a film of liquid metalinterposed between said electrode structure and said supportingstructure.
 2. The combination of claim 1 in which the liquid metal is analloy having a low vapor pressure at the normal operating temperature ofthe device.
 3. The combination of claim 2 in which the liquid metal isan alloy which is liquid at ambient temperature.
 4. The combination ofclaim 2 in which the liquid metal has a low viscosity.
 5. Thecombination of claim 2 in which said liquid metal is an alloy ofgallium, indium and tin.
 6. The combination of claim 2 in which saiddevice is an X-ray tube, said electrode structure comprises an anode anda supporting shaft, and said supporting structure contains a passagewayfor receiving a cooling fluid from a source external to said device. 7.The combination of claim 6 in which a bearing is interposed between saidrotating electrode structure and said supporting structure and saidliquid metal comprises a material which does not alloy readily with thematerials of said bearing and said rotating structure.
 8. Thecombination of claim 7 in which the liquid metal comprises an alloy ofgallium, indium and tin, said bearings are ball bearings and are formedof material from the group consisting of steel and aluminum oxide. 9.The combination of claim 6 in which said supporting structure includes acentrally positioned hollow cylinder and means for providing a coolingfluid to the interior of said cylinder.
 10. The combination of claim 6in which said supporting structure includes a centrally positionedhollow cylinder and heat pipe means are included in said hollowcylinder.
 11. The combination of claim 6 in which said supportingstructure has a closed end which is opposed to said anode to form aregion therebetween for receiving liquid metal, said supporting shafthas a liquid metal trap on the inner surface thereof spaced from saidsupporting structure and said supporting structure includes a passagewayfor returning liquid metal from said closed region to said trap.
 12. Themethod of conducting heat from a moving electrode subject to heating byan electron beam in an evacuated enclosure which comprises interposing afilm of liquid metal between stationary and moving parts of a thermalpath between the moving electrode and a stationary metal surface andcooling the stationary metal surface.
 13. The method of claim 12 inwhich the moving electrode is the anode of an X-ray tube, the movingpart is a shaft supporting the anode, the stationary part is a bearingstructure for the shaft, and which includes the steps of interposing afilm of liquid metal between the shaft and the bearing structure,trapping any liquid which escapes from the region between the shaft andbearing structure during non-use periods of the tube and returning it tothe region during periods of use.